Fusion Definition (Physics and Chemistry)

The Different Meanings of Fusion in Science

Fusion releases tremendous amounts of energy, but only if the resulting nuclei are light.
Fusion releases tremendous amounts of energy, but only if the resulting nuclei are light. aleksandarnakovski / Getty Images

The term "fusion" refers to key concepts in science, but the definition depends on whether that science is physics, chemistry, or biology. In its most general sense, fusion refers to synthesis or to the joining of two parts. Here are the different meanings of fusion in science:

Key Takeaways: Fusion Definition in Science

  • Fusion has several meaning in science. In general, they all refer to the joining of two parts to form a new product.
  • The most common definition, used in physical science, refers to nuclear fusion. Nuclear fusion is the combination of two or more atomic nuclei to form one or more different nuclei. In other words, it is a form of transmutation that changes one element into another.
  • In nuclear fusion, the mass of the product nucleus or nuclei is lower than the combined mass of the original nuclei. This is due to the effect of binding energy within the nuclei. Energy is required to force the nuclei together and energy is released when new nuclei form.
  • Nuclear fusion may be either an endothermic or exothermic process, depending on the mass of the initial elements.

Fusion Definitions in Physics and Chemistry

  1. Fusion means combining lighter atomic nuclei to form a heavier nucleus. Energy is absorbed or released by the process and the resulting nucleus is lighter than the combined masses of the two original nuclei added together. This type of fusion may be termed nuclear fusion. The reverse reaction, in which a heavy nucleus splits into lighter nuclei, is called nuclear fission.
  2. Fusion may refer to the phase transition from a solid to a light via melting. The reason the process is called fusion is because the heat of fusion is the energy required for a solid to become a liquid at that substance's melting point.
  3. Fusion is name of a welding process used to join two thermoplastic pieces together. This process may also be called heat fusion.

Fusion Definition in Biology and Medicine

  1. Fusion is the process by which uninuclear cells combine to form a multinuclear cell. This process is also known as cell fusion.
  2. Gene fusion is the formation of a hybrid gene from two separate genes. The event may occur as the result of a chromosomal inversion, translocation, or interstitial deletion.
  3. Tooth fusion is an abnormality characterized by the joining of two teeth.
  4. Spinal fusion is a surgical technique that combines two or more vertebrate. The procedure is also known as spondylodesis or spondylosyndesis. The most common reason for the procedure is to relieve pain and pressure on the spinal cord.
  5. Binaural fusion is the cognitive process through which auditory information from both ears is combined.
  6. Binocular fusion is the cognitive process through which visual information is combined from both eyes.

Which Definition to Use

Because fusion can refer to so many processes, it's a good idea to use the most specific term for a purpose. For example, when discussing the combination of atomic nuclei, it's better to refer to nuclear fusion rather than simply fusion. Otherwise, it's usually obvious which definition applies when used in the context of a discipline.

Nuclear Fusion

More often than not, the term refers to nuclear fusion, which is the nuclear reaction between two or more atomic nuclei to form one or more different atomic nuclei. The reason the mass of the products is different from the mass of the reactants is due to the binding energy between atomic nuclei.

If the fusion process results in a nucleus lighter in mass than the isotopes iron-56 or nickel-62, the net result will be an energy release. In other words, this type of fusion is exothermic. This is because the lighter elements have the largest binding energy per nucleon and the smallest mass per nucleon.

On the other hand, fusion of heavier elements is endothermic. This may surprise readers who automatically assume nuclear fusion releases a lot of energy. With heavier nuclei, nuclear fission is exothermic. The significance of this is that heavier nuclei are much more fissionable than fusible, while lighter nuclei are more fusible than fissionable. Heavy, unstable nuclei are susceptible to spontaneous fission. Stars fuse lighter nuclei into heavier nuclei, but it takes incredible energy (as from a supernova) to fuse nuclei into elements heavier than iron!